An optical navigation device, such as an optical mouse, typically includes a navigation sensor that functions as a miniature digital camera to continually collect images of a surface that the device is sitting on and to determine the speed and direction that the device is being moved across the surface by comparing sequentially recorded frames of image information. Image frames are collected at a very high rate, such as 1,500 image frames per second, and the resolution of the navigation sensor is high enough to detect very small movements of the device relative to the surface.
Conventional optical navigation devices utilize a light emitting diode as the light source to illuminate the surface upon which the device sits. Next generation optical navigation devices are utilizing a vertical cavity surface emitting laser (VCSEL), which emits coherent light in the infrared (IR) spectrum (e.g., at about 830-865 nm), as the light source to illuminate the surface. Utilizing a VCSEL that generates coherent light in the IR spectrum as the light source in an optical mouse provides many advantages, including allowing the optical mouse to work on surfaces with fewer imageable features and providing higher resolution navigation than conventional LED-based devices.
Because the spectrum of visible light ranges from approximately 400-700 nm, the light used in conventional VCSEL-based optical navigation devices is invisible to the human eye. A laser beam generated from a VCSEL can be a safety hazard, especially if the beam is directed into the human eye. The danger of exposure to an invisible laser beam is enhanced by the fact that the beam cannot be seen.
In order to mitigate the danger of a laser beam used in an optical navigation device such as an optical mouse, the beam intensity of each VCSEL is measured and calibrated, if necessary, to ensure that the beam intensity does not exceed an intensity limit. However, because the beam is invisible, it is difficult to ensure that the beam output from an optical navigation device is accurately aligned with the measurement sensor.